Bis(acetylacetonato)ruthenium(II) complexes containing bulky tertiary phosphines. Formation and redox behaviour of Ru(acac)2 (PR3) (R = iPr, Cy) complexes with ethene, carbon monoxide, and bridging dinitrogen

Abstract Reaction of cis -[Ru(acac) 2 (η 2 -C 8 H 14 ) 2 ] ( 1 ) (acac = acetylacetonato) with two equivalents of P i Pr 3 in THF at −25 °C gives trans -[Ru(acac) 2 (P i Pr 3 ) 2 ], trans - 3 , which rapidly isomerizes to cis - 3 at room temperature. The poorly soluble complex [Ru(acac) 2 (PCy 3 ) 2 ] ( 4 ), which is isolated similarly from cis -[Ru(acac) 2 (η 2 -C 2 H 4 ) 2 ] ( 2 ) and PCy 3 , appears to exist in the cis -configuration in solution according to NMR data, although an X-ray diffraction study of a single crystal shows the presence of trans - 4 . In benzene or toluene 2 reacts with P i Pr 3 or PCy 3 to give exclusively cis -[Ru(acac) 2 (η 2 -C 2 H 4 )(L)] [L = P i Pr 3 ( 5 ), PCy 3 ( 6 )], whereas in THF species believed to be either square pyramidal [Ru(acac) 2 L], with apical L, or the corresponding THF adducts, can be detected by 31 P NMR spectroscopy. Complexes 3 – 6 react with CO (1 bar) giving trans -[Ru(acac) 2 (CO)(L)] [L = P i Pr 3 ( trans - 8 ), PCy 3 ( trans - 9 )], which are converted irreversibly into the cis -isomers in refluxing benzene. Complex 5 scavenges traces of dinitrogen from industrial grade dihydrogen giving a bridging dinitrogen complex, cis -[{Ru(acac) 2 (P i Pr 3 )} 2 (μ-N 2 )] ( 10 ). The structures of cis - 3 , trans - 4 , 5 , 6 and 10  · C 6 H 14 have been determined by single-crystal X-ray diffraction. Complexes trans - and cis - 3 , 5 , 6 , cis - 8 , and trans - and cis - 9 each show fully reversible one-electron oxidation by cyclic voltammetry in CH 2 Cl 2 at −50 °C with E 1/2 (Ru 3+/2+ ) values spanning −0.14 to +0.92 V (versus Ag/AgCl), whereas for the vinylidene complexes [Ru(acac) 2 (C CHR)(P i Pr 3 )] [R = SiMe 3 ( 11 ), Ph ( 12 )] the process is irreversible at potentials of +0.75 and +0.62 V, respectively. The trend in potentials reflects the order of expected π-acceptor ability of the ligands: P i Pr 3 , PCy 3 2 H 4 CHR  III –ethene cation 6 + has been observed at −50 °C. Cyclic voltammetry of the μ-dinitrogen complex 10 shows two, fully reversible processes in CH 2 Cl 2 at −50 °C at +0.30 and +0.90 V (versus Ag/AgCl) corresponding to the formation of 10 + (Ru II,III ) and 10 2+ (Ru III,III ). The former, generated electrochemically at −50 °C, shows a band in the near IR at ca. 8900 cm −1 ( w 1/2 ca. 3700 cm −1 ) consistent with the presence of a valence delocalized system. The comproportionation constant for the equilibrium 10  +  10 2+  ⇌ 2 10 + at 223 K is estimated as 10 13.6 .